Supernovae in Aspen

Artist's impression of a Type Ia Supernova
explosion. A white dwarf (left) steals the matter from the
outer envelopeof a red giant companion (right) until it can
no longer support the weight. The resulting explosion
can be seen across the universe. Credit: David A. Hardy,
www.astroart.org & PPARC

I'm writing this from the plane on a flight back to SFO from Colorado. I just spent the last week in Aspen at the Center for Physics in a supernova workshop. Yes, you heard right, we actually have workshops on supernovae!

You may wonder how it is that a group of people could spend an entire week discussing exploding stars. Well, I'll tell you, there's a lot we don't know about them. If you ask around, you'll find astrophysicists who have been studying supernovae for their entire professional careers, and who still don't have it figured out.

Before I start with all the things we don't yet know, let me tell you a couple of the things we do know. First of all, as I mentioned above, a supernova is an exploding star. Roughly speaking, there are two classes, the core collapse and the Type Ia.

A large star (eight times more massive than the Sun) will die in a core collapse event when it burns off the last of its fuel. Without any fuel to burn, the nuclear explosions at the center stop firing, and no longer support the star against its own gravity. When this happens, the star implodes in a burst of light, energy and heavy elements, leaving behind a neutron star or a black hole, depending on the size of the original star.

A Type Ia supernova is a little less intuitive. This type of event actually requires two stars in a very specific kind of binary system. One of these stars is a red giant with a very large radius and a loosely confined atmosphere. The other star is a white dwarf, a star made up largely of carbon and oxygen with no fuel left to burn. A white dwarf can support its own weight only up to a mass a little larger than that of our Sun. This mass limit is known as the Chandrasekhar Mass.

The white dwarf is much more dense than its companion (one teaspoon of it would weigh a ton) and yanks the material away from its companion. The process is an extreme version of the way the moon causes tides on Earth. As the white dwarf collects this material, it eventually grows larger than the Chandrasekhar Mass. Once it grows so large, it cannot support its own weight anymore, and implodes, as shown in this computer simulation. Type Ia supernovae therefore explode at a very well defined mass and composition. Unlike core collapse supernovae, most Type Ia events look very similar to each other.

What I just painted was a very simple picture of the supernovae that we observe. When it comes to the details however, there are some great mysteries. For example, we don't know where the explosions start or how the explosions propagate through the star. We have some pretty good ideas, but we don't know the details of why Type Ia events actually differ by 20% or so, or how often supernovae go off in the Milky Way, much less a galaxy 10 billion light years away.

These are some of the issues we came to Aspen to discuss. I imagine that with improved telescopes and computer simulations, we will make some serious progress toward understanding the details of supernovae in the next ten years. I just hope no one figures it out too quickly, since I really enjoyed this weeklong trip to the Colorado Rockies.

Kyle S. Dawson is engaged in post-doctorate studies of distant supernovae and
development of a proposed space-based telescope at Lawrence Berkeley National Laboratory.